To interact successfully with the local environment, people must maintain a "local map' of the locations of important nearby objects. This is one of the primary functions of spatial working memory. The majority of research investigating the characteristics of spatial working memory has focused on what people represent in memory. Although this is a fundamental question, the present study examines a different, but equally important question: how do people maintain location information in memory? This question is timely because recent studies have identified several brain regions that contribute to the maintenance of location information. Furthermore, studies have demonstrated that elderly participants and patients suffering from Alzheimer's Disease show deficits when the characteristics of a visual stimulus must be maintained in working memory over a delay. Evidence from a variety of studies suggests that people rely on visible reference axes -- the edges of a table, the edges of a computer screen -- to help them remember the locations of target objects. Use of such perceptual information can help stabilize memory. Nevertheless, there is a cost -- memory is systematically distorted near reference axes. Specifically, when people are asked to reproduce the location of a hidden object, they exaggerate the distance between the reference axis and the actual location of the object. These memory errors are particularly informative because they increase in magnitude as memory delays increase. Thus, errors away from reference axes may provide a window into the second-to-second processes the serve to maintain location information in working memory. Fourteen experiments will test a mathematical model of how information is maintained in spatial working memory with both child and adult participants. The first eight experiments test specific predictions of the model. The final seven experiments examine the generality of the model to situations in which a reaching response is not required and to situations that place demands on spatial selective attention. When completed, the proposed project will provide the first formal theory of spatial working memory.